Microelectronic imagers are used in digital cameras, wireless devices with picture capabilities, and many other applications. Cell phones and Personal Digital Assistants (PDAs), for example, are incorporating microelectronic imagers for capturing and sending pictures. The growth rate of microelectronic imagers has been steadily increasing as they become smaller and produce better images with higher pixel counts.
Microelectronic imagers include image sensors that use Charged Coupled Device (CCD) systems, Complementary Metal-Oxide Semiconductor (CMOS) systems, or other solid state systems. CCD image sensors have been widely used in digital cameras and other applications. CMOS image sensors are also quickly becoming very popular because they are expected to have low production costs, high yields and small sizes. CMOS image sensors can provide these advantages because they are manufactured using technology and equipment developed for fabricating semiconductor devices. CMOS image sensors, as well as CCD image sensors, are accordingly “packaged” to protect the delicate components and to provide external electrical contacts.
FIG. 1 is a schematic view of a conventional microelectronic imager 1 with a conventional package. The imager 1 includes a die 10, an interposer substrate 20 attached to the die 10, and a spacer 30 attached to the interposer substrate 20. The spacer 30 surrounds the periphery of the die 10 and has an opening 32. The imager 1 also includes a transparent cover 40 over the die 10.
The die 10 includes an image sensor 12 and a plurality of bond-pads 14 electrically coupled to the image sensor 12. The interposer substrate 20 is typically a dielectric fixture having a plurality of bond-pads 22, a plurality of ball-pads 24, and traces 26 electrically coupling bond-pads 22 to corresponding ball-pads 24. The ball-pads 24 are arranged in an array for surface mounting the imager 1 to a board or module of another device. The bond-pads 14 on the die 10 are electrically coupled to the bond-pads 22 on the interposer substrate 20 by wire-bonds 28 to provide electrical pathways between the bond-pads 14 and the ball-pads 24.
The imager 1 shown in FIG. 1 also has an optics unit including a support 50 attached to the transparent cover 40 and a barrel 60 adjustably attached to the support 50. The support 50 can include internal threads 52, and the barrel 60 can include external threads 62 engaged with the threads 52. The optics unit also includes a lens 70 carried by the barrel 60.
One aspect of fabricating the imager 1 is forming the spacer 30 and attaching the cover 40 to the spacer 30. The spacer 30 can be formed by placing an uncured, flowable epoxy onto the interposer substrate 20. In a typical application, the interposer substrate 20 has a plurality of separate dies 10, and the spacer 30 is formed as a grid of uncured epoxy on the interposer substrate 20 in the areas between adjacent dies 10. After depositing the epoxy, the cover 40 is attached to the spacer 30. The epoxy is then cured to harden the spacer 30 such that it becomes dimensionally stable after enclosing the die 10 between the cover 40 and the interposer substrate 20.
One problem of forming the spacer 30 by stenciling an uncured epoxy on the interposer substrate is that the stenciling process produces a textured surface on the top surface of the spacer 30. This can lead to leaks between the spacer 30 and the cover 40 through which moisture or other contaminants can enter into the cavity where the image sensor 12 is located. Another problem of forming the spacer 30 by stenciling an uncured epoxy onto the substrate is that the height of the spacer 30 is limited because the epoxy tends to slump after the stencil is removed. This causes the epoxy to flow laterally and occupy a significant percentage of the real estate on the substrate 20. Additionally, a significant problem of using an uncured epoxy is that the uncured epoxy outgases during the curing cycle after the cover is mounted to the epoxy. Such outgasing can contaminate the compartment and impair or ruin the performance of the die 10.
Another process for forming the spacer 30 is to dispense a small flow of uncured epoxy via a needle-like tube or nozzle between adjacent dies. This process is undesirable because it is difficult to control the flow of the uncured epoxy at the intersections of the grid. The intersections typically have rounded corners that occupy additional real estate on the interposer substrate. Additionally, as with the stencil printing process, the epoxy is cured after the cover 40 is mounted to the spacer 30 such that it outgases into the image sensor compartment. Therefore, processes that dispense an epoxy using needle-like tubes are also undesirable.
U.S. Pat. No. 6,285,064 discloses another process in which a preformed adhesive matrix is fabricated in the shape of a wafer. The adhesive matrix has openings in the pattern of the image sensors, and it is formed separately from the wafer. In operation, the adhesive matrix is attached to the wafer such that the openings are aligned with the microlenses, and a cover glass is then attached to the top of the adhesive matrix. The adhesive matrix is subsequently activated by application of light, pressure and/or heat.